An anesthetic machine is auxiliary equipment for guiding a patient to inhale anesthetic gas and for mechanical ventilation during an operation. Flow and airway pressure are the most basic parameters which the anesthetic machine monitors, and the calculation of other parameters or even the whole mechanical ventilation process control is based thereon. Therefore, the precision and stability in measuring and testing these two parameters determines not only the accuracy and safety of the anesthetic machine in controlling the mechanical ventilation but also the ventilation modes that the anesthetic machine adopts and the performance that the anesthetic machine can achieve. The present invention is related mainly to the monitoring and controlling of the flow, the more important and also more difficult parameter to be accurately monitored.
Depending on the locations where the measurements are made, prior art methods for monitoring flows in the anesthetic machine are classified as the patient end monitoring and the anesthetic machine end monitoring.
The patient end monitoring is also referred to as the proximal end monitoring, wherein the flow monitoring point is located close to the respiratory tract of the patient. This type of monitoring has found very wide application in the currently available products, which is mainly executed in the following two forms: 1) a flow sensor is installed downstream of the Y-shaped connector in the patient breathing circuit for simultaneously monitoring the velocity of the exhaled and inhaled gas; 2) two flow sensors are installed in the exhaling conduit and the inhaling conduit of the patient breathing circuit respectively for separately monitoring the velocity of the exhaled and inhaled gas. These two forms are all directed to the measurement of the flow velocity in the patient breathing circuit. Since the measuring point is located near the respiratory tract of the patient, the influence of the leakage from conduits and the change of compliance on the measurement can be reduced remarkably, so that the measured value is substantially equal to the gas flow that is delivered actually into the patient's lungs. High measuring accuracy and sensitivity are thereby ensured. However, the same fact that the measuring point is located near the respiratory tract of the patient also produces unfavorable effect, because in this case the flow sensor is affected easily by the secretions of the respiratory tract of the patient and the water vapor condensate in the respiratory conduit such that the measured results may deviate from the true values and that the ventilation control may be disabled.
The anesthetic machine end monitoring is also referred to as the distal end monitoring, wherein the flow monitoring point is located inside the anesthetic machine and far away from the respiratory tract of the patient. The flow sensor is usually installed following the inhalation valve inside the anesthetic machine for monitoring the flow of the drive gas flowing out of the inhalation valve. The flow of the drive gas within one breathing cycle is regarded as equal to the tidal volume inhaled by the patient. Since the drive gas flowing through the flow sensor is dry, the measured value is free from the influence of the water vapor condensate and the secretions in the patient breathing circuit. However, since it is the flow of the drive gas that is measured, which is influenced by the leakage from conduits and the change of compliance, this measured flow deviates from the actual ventilation volume in certain degree. Moreover, this deviation can not be compensated because the amount of the leakage and the extent of the change of compliance in the breathing circuit can not be monitored. In addition, with respect to the realization of some respiratory functions, flow speed trigger and the like can not be realized because the flow sensor is located far away from the patient end. As a result, the distal end monitoring is vanishing from the market.
The closed-loop flow control means to control the opening of the inhalation valve by using the measured value of the flow sensor as the feedback signals and thus control the ventilation volume for the patient. Specifically, the measured value of the flow sensor is compared with a preset value, and if the two values are inconsistent, the microprocessor will adjust the opening of the inhalation valve according to the deviation between the two values until the two values reach consistency. Regardless of a patient end flow monitoring or an anesthetic ventilator end flow monitoring, this control mode has a serious defect. When the measured value of the flow sensor largely deviates from the preset value, there is no way to determine the causes for the deviation, for example, whether it is caused by the inaccurate measurements of the flow sensor or resulted from changes in the characteristics of the inhalation valve. To make the parameters concerning the flow monitoring consistent with the preset parameters, it is generally only possible to adjust the inhalation valve against the measured value of the sensor. In this way, the user is unable to find out the deviation in the sensor measurements. As a further result, the actual ventilation volume supplied to the patient is not consistent with the predetermined volume and so it is not possible to ensure the safety of the mechanical ventilation. In order to ensure the patient's safety, a common prior solution is to use the characteristic curve of the inhalation valve as the basis for the judgment. When the deviation between the ventilation volume derived from the characteristic curve and the value measured with the flow sensor exceeds the predetermined threshold value, but at the same time the monitored result of the inhalation valve control circuit shows that the inhalation valve works normally, the anesthetic machine will judge immediately that a sensor failure has occurred and will suspend the flow monitoring. Where the anesthetic machine continues its operation, it can only depend on the inhalation valve itself to control the flow. As such, due to lack of flow monitoring, the influence of the inhalation valve on the ventilation volume arisen from the control circuit drift and the change in the own characteristics as well as the leakage from conduits and the compliance is unable to be eliminated. As a result, a rather serious deviation between the actual ventilation volume and the predetermined value may still exist. Therefore, the accuracy and safety in using an anesthetic machine to apply mechanical ventilation to the patient can still hardly be guaranteed.
To sum up, the prior technology for monitoring and controlling the flow in an anesthetic machine is subjected to the following disadvantages:
1. The distal end monitoring can not guarantee that the monitored ventilation volume corresponds to the patient's actual inhaling volume, and moreover, it can not realize flow trigger; on the other hand, though the proximal end monitoring achieves relatively high precision and sensitivity, it often gives rise to greater measurement deviation and even causes the measurements to be invalidated when the flow sensor is exposed to heavy water vapor condensation within the respiratory conduit in the course of use.
2. By using single proximal end or distal end flow measurement method, when deviation or invalidation appears in the flow sensor or in the inhalation valve, the system can only adjust one against the other. This kind of treatment mechanism can not guarantee the accuracy of the ventilation control. Moreover, the safety of the mechanical ventilation can not be guaranteed when great deviations occur to the benchmark, and it is even possible to cause injuries to the patient.
The above-mentioned problems also exist in other medical equipment including ventilators which are used to influence a patient's respiratory system. Among them, the safety problem of an anesthetic machine is particularly concerned.